Chemical Properties
Usually, Wittig reagents are prepared in situ from the appropriate phosphonium salt. As such, benzyltriphenylphosphonium chloride is available as a white powder or as a mixture with sodium amide, NaNH2. benzyltriphenylphosphonium chloride is a slightly hygroscopic solid. It can be dried efficiently under vacuum at elevated temperature (<10 Torr and 40 °C) overnight. Benzylidenetriphenylphosphorane is destroyed rapidly by exposure to oxygen or to moisture, and for this reason is usually prepared immediately before use. When protected from these materials, however, solutions of the ylide are stable for extended periods of time.
Synthesis
Benzylidenetriphenylphosphorane (Phosphorane,triphenyl(phenylmethylene)-) is prepared from benzyltriphenylphosphonium chloride by deprotonation with base. Formation of the ylide from this salt has been accomplished with many different base and solvent combinations. In more recent preparations the salt typically is suspended in dry THF under an inert atmosphere and a solution of the base, either BuLi or KHMDS, is added by syringe. Alternatively, the salt and NaNH are transferred to a flask in a dry box and then benzene is added. The ylide can also be generated with alkoxide bases in protic solvents (such as NaOEt/EtOH) or under phase transfer conditions (CH2Cl2/H2O with NaOH).
References
1. (a) Maryanoff, B. E.; Reitz, A. B. CRV 1989, 89, 863. (b) Gosney, I.; Rowley, A. G. In Organophosphorus Reagents in Organic Synthesis; Cadogan, J. I. G., Ed.; Academic: New York, 1979. (c) Schlosser, M. Top. Stereochem. 1970, 5, 1. (d) Reucroft, J.; Sammes, P. G. QR 1971, 25, 135.
2. Johnson, A. W. Ylid Chemistry; Academic: New York, 1966.
3. References with detailed experimental procedures: (a) BuLi: Ward, W. J., Jr.; McEwen, W. E. JOC 1990, 55, 493. (b) KHMDS: Vedejs, E.; Marth, C. F.; Ruggeri, R. JACS 1988, 110, 3940.
4. (a) Schlosser, M.; Christmann, K. F. LA 1967, 708, 1. (b) House, H. O.; Jones, V. K.; Frank, G. A. JOC 1964, 29, 3327.
5. Allen, D. W. JCR(S) 1980, 384.
6. M?rkl, G.; Merz, A. S 1973, 295.
